The present invention relates to a measuring apparatus which emits an ultrasonic beam to a medium under test and which utilizes the reflected wave therefrom in medical diagnosis, nondestructive testing and so forth and, more particularly, to an utlrasonic sector-scan probe which emits an ultrasonic beam fanwise from its portion held in contact with the medium under test for observing its interior.
The sector scan of an ultrasonic beam has the advantage that the interior of a living tissue, for example, can be observed over a wide field from one small contact portion. Especially, for observing a heart or the like, the sector scan is exclusively employed because an appreciable area of the heart can be observed by contacting a probe having a narrow portion between pairs of ribs, for avoiding the obstacles presented by the ribs.
The sector-scan techniques heretofore employed are roughly divided into a mechanical scan method which drives a focused utlrasonic beam to scan fanwise by mechanically wobbling or rotating a disc-shaped or rectangular, concave, piezoelectric transducer, and a phased array type electronic scan method in which a number of narrow rectangularly-shaped piezoelectric segments are arrayed, wherein the temporal phases for driving the elements for transmission are controlled and received signals are also controlled in their temporal phases and added together, thereby to electronically deflect the directions of transmission and reception.
The mechanical scan method is highly advantageous in that it is simple, inexpensive and excellent in beam performances, such as in the beam directivity pattern, intensity and frequency spectrum and such as in the direction/magnitude of a side lobe, and in that these beam performances do not vary with the angle of deflection. With this method, however, owing to mechanical inertia, the scanning must be effected in a sequential order and cannot be jumped nonsequentially to a desired scanning line.
In contrast thereto, the phased array type electronic scan method permits such an arbitrarily jumping scan but necessitates the complex and bulky circuit arrangements for phase control, and hence is very expensive. Also, it is inferior in the beam performances to the mechanical scan method, and the abovesaid performance of the beam undergoes substantial changes with the angle of deflection. Furthermore, this method possesses the defect that a grating side lobe resulting from an array construction exerts a bad influence upon measurement.
In FIG. 1, 3-1 shows a system proposed by Aloka Limited (Japanese Utility Model application publication No. 41267/77) and 3-2 shows a system proposed by Hoffman la Roche Ltd. (Japanese Patent application publication No. 41267/77) for obviating the aforementioned drawbacks. In FIG. 1, reference numeral 1 indicates linearly arrayed ultrasonic transducer segments on a circular arc; 2 designates the intersection point of scanning lines; 3-1 and 3-2 identify apertures formed of corresponding segment groups; 4 denotes an acoustic window; and 10 represents a front space or room. In this system, the scanning is performed by shifting the location of the operated group in the arrayed segments. The linear array of transducer segments that is well known for abdomen diagnosis is rearranged on an arcuate, concave and circular arc and the focussing point of the beam provided by the aperture comes to the center of the circular arc in the case of 3-1, which is set on the contacting window of the probe with the member under test or in its vicinity. The beam scanning is effected by simple change-over control of the segments instead of by phase control, so that this system is simple and inexpensive. Besides, although the grating lobe cannot be removed, the beam performances are excellent and have no deflection angle dependence. In this system, however, the piezoelectric segments forming the transducer array 1 and the window 4 which contacts the member under test, such as the human body or the like, for transmitting thereinto ultrasonic waves and receiving therefrom reflected waves, must be spaced apart an equal distance to the radius of curvature of the circular arc, and the space defined between the array 1 and the window 4 (which space will hereinafter be referred to as the front room 10) must be filled up with a medium that conducts ultrasonic waves. Furthermore, in order to avoid the influence of what is called multiple reflection, that arises from re-reflection and re-radiation of reflected waves between the window 4 or the skin surface and the piezoelectric array, it is necessary to make substantially equal the propagation time of ultrasonic waves in the front room 10 and the propagation time of ultrasonic waves in a range to be measured in the member under test. According to the above-mentioned prior art, in the case where water is used as the medium filling up the front room 10, since the speeds of sound in water and in the living tissue are nearly equal to each other, the path length in the front room, that is, the radius of curvature of the circular arc of the concave array, must be almost the same as the range to be measured (about 18 cm in the case of a heart or the like). Accordingly, the probe used is very bulky. The overall angle of scanning in the front room 10 (equal subtended at the angle to the center of curvature by the circular arc) becomes equal to the overall angle of scanning in the living body (90.degree. in the case of a heart or the like). In consequence, the angle of the front end of the probe exterior, which probe includes its container and a sound absorber, becomes almost 100.degree., making it difficult to incline the direction of the center of the fan-wise scanning between the ribs. Moreover, the beam pattern is defined only by the aperture defined by the group 3-1; namely, the beam diameter decreases, from that of the aperture formed by the segment group 3-1, towards the center of curvature of the circular arc, is a minimum at the center of curvature and then increases point-symmetrically with respect to the center of curvature. At the deepest range in the member under test, the beam diameter is substantially equal to the diameter of the aperture. Accordingly, the degree of convergence of the beam is very low. To obviate such a defect, it has been proposed by Hoffman la Roche Ltd. as noted above to perform focussing by the aperture segment group 3-2 in FIG. 1 electronically. The focus point of the scanning beam formed by the aperture of segment group 3-2 is not limited specifically to the vicinity of the window 4, unlike the case of the scanning beam formed by the aperture of segment group 3-1. By controlling the phases of driving for each segment in the group and the phases of reception, the convergence of the beam is electronically weakened so that the focus point, which in the case of 3-1 is geometrically set to the center of curvature of the circular arc, can be moved to a farther position, for instance, at a position two-thirds of a maximum depth of measurement (indicated by O.sub.1), thereby resulting in better beam convergence throughout the range. As will be appreciated from FIG. 1, however, in the system 3-1 above proposed by Aloka Limited, since the beam diameter at the window 4 is sufficiently reduced, the width of the window 4 can be made small, whereas in the above system 3-2 proposed by Hoffman la Roche Ltd. the beam diameter at the window 4 is large and the width of the window 4 cannot be made small. This is disadvantageous for the sector scan which is mostly used for diagnosing a heart from a narrow gap between the ribs.
In practice, there exists, in addition to the beams shown in FIG. 1 which can be handled by acousto optics, a diffusive beam which is to be superimposed on the abovesaid beams and which linearly spreads out at a vertex angle of 2 times 0.6 .lambda./a radian, where a is the half-width of an aperture of the group and .lambda. is the wavelength of the ultrasonic waves used. This is common to both systems described above in respect of FIG. 1, and the width of this beam is larger than or substantially equal to the width of the acousto-optical beam in the focal and farther regions, but no consideration is paid to this beam in either prior art.
In the Hoffman la Roche Ltd. prior art above there is further proposed a method of permitting scanning at a one-half pitch of the segment pitch and a method of beam focusing in a perpendicular direction to the sector scan plane. Since such methods are well-known in the conventional linear array of a probe for diagnosing the abdomen, no further description will be given hereafter. In this prior art it is further proposed to use, as the medium in the front room, a medium in which the speed of sound is lower than in water. This makes it possible to obviate the defect that the probe proposed in the aforementioned Japanese Utility Model application publication No. 4126/77 of the first prior art system 3-1 above is very bulky. The media specified in this prior art include certain biological liquids and silicone rubber, the speed of sound in which is about 1000 m/sec. Since the speed of sound in water and the living body is approximately 1500 m/sec, the path length in the front room above is about 2/3 that of this prior art, and the size of the probe and the angle of scan can be reduced to substantially 2/3 those in the utility model gazette. From a practical point of view, however, the measurement depth range is 18 cm and the overall angle of scanning is 90.degree., for instance, in diagnosing a heart. However, with the probe of Hoffman la Roche Ltd. above the path length in the front room 10 can be 12 cm and the overall angle of scanning is about 60.degree., while the probe, including the sound absorber and the container wall, is still too large for practical use. This is understood to be one reason why the prior art system 3-2 above has not provided in in a practical system.
FIG. 2 illustrates the state of applying an ultrasonic fan-shaped probe in a tilted condition. Reference numeral 1 indicates a linear array; 2 designates the intersection of scanning lines; 7 identifies a sound absorber and container; 8 denotes the surface of the body (or the skin surface); and 9 represents an internal organ near the skin surface. In view of the length of the prior art teaching of the probe above, the 12 cm path length in the front room 10 is sufficient for practical use but the 60.degree. overall angle of scanning in the front room 10 is too large. That is, as shown in FIG. 2, in order to observe the organ 9 (for example, the right atrium or the right ventricle of the heart) near the skin surface over a wide visual field, it is desirable to tilt the fan-shaped probe, so that the most deflected scan line almost coincides with the skin surface 8 as shown. In this case, the center line l of the probe forms an angle .theta..sub.2 with the skin surface 8 and the angle of scanning in the front room is .theta..sub.1 on either side of the center line l. Accordingly, a marginal angle .alpha., for tilting .alpha. with respect to the skin surface 8, is .theta..sub.2 -.theta..sub.1. In practice, an additional angle other than .theta..sub.1 is also involved, which arises due to the sound absorber 7, the container and the marginal room around the fan-shaped space in the front room 10. Consequently, it is necessary that .theta..sub.2 -.theta..sub.1 be larger than 20.degree.. In the case where 2.theta..sub.2 =90.degree. and 2.theta..sub.1 =60.degree., the marginal angle .alpha. for tilting is .theta..sub.2 -.theta. .sub.1 =15.degree., which is insufficient for practical use.
Furthermore, since only one tomographical section is obtained with the prior-art ultrasonic sector-scan probes, no accurate geometrical orientation with respect to an organ can be effected. For instance, the heart, always pulsates and also shifts and rotates three-dimensionally as a whole owing to breathing. Accordingly, with the observation of only one section, it is unclear which part of the heart is scanned. To solve this problem, it is customary in the prior art to make observations of desired sections in sequence while changing the posture of the probe with respect to the examinee's body and while turning the probe by 90.degree. around its axis. In this case, since the probe is manually rotated, it is very difficult to retain geometrical accuracy with each change of the posture of the probe.
It is considered that this problem could be solved by mechanically holding the probe in a fixed posture. However, this prevents breathing and hence is undesirable, and it has not been employed at all.
Another solution heretofore proposed is to mechanically turn the probe by 90.degree. around its axis while manually holding its outer container in a fixed posture with respect to the examinee's body. But this method is disadvantageous in that the probe used is bulky, and in that measurement errors are still large because it is difficult to manually hold the probe container in a completely fixed posture, because different sections are not concurrently observed and because much time is needed for rotating the probe.
Apart from the present invention, the present inventor has proposed a method of observing two sections simultaneously with two probes while at the same time detecting their relative positions with angle detectors mounted on the joints of their linkage arm, thereby to enable three-dimensional but accurate observations.
In this case, however, two probes are used and they must contact with the examinee's body at two places. Generally, in the cases of middle-aged or younger persons, two contact areas effective for the examination of the heart can be found but, as for aged persons, only one effective contact area can be found in many cases, and the successful cases involve 40 to 50% of examinees of all ages.
In view of the above, there is a strong demand for the realization of a system for electronically switching two or more different sections at high speed or for observing them simultaneously through one contact area, but such a system has not been reported yet.
Usually the electronic sector scan is performed using a phased array. For this sector scan, there has also been proposed by the present inventor to arrange two phased arrays in laminated layers for scanning perpendicular sections so that each sector scan plane perpendicularly intersects the other, for instance. In this case, the individual sections can be observed simultaneously be using different frequencies for them.
However, the fabrication of such a double-layer phased array is complex and the phase control circuit is also complex and expensive.